1. Field of the Invention
The present invention relates to correcting a soft decision value used for decoding of an error correcting code used in a receiver.
2. Description of the Related Art
A digital wireless communication system that transmits and receives data among a plurality of communication units using wireless communication is previously known. The carrier sense multiple access with collision avoidance (CSMA/CA) method is known as one of the access control systems used in the digital wireless communication systems.
In accordance with a typical CSMA/CA access control method, a communication device that intends to transmit data performs carrier sensing on a channel to be used for communication with a communication device at reception destination. If no carrier is sensed, indicating that the channel is not busy, the communication device transmits the data. If the presence of a carrier is sensed, indicating that the channel is busy, the communication device waits until no carrier is sensed, indicating that the channel is not busy, before transmitting the data.
In a typical wireless communication system, a convolutional code is one of the known coding systems for encoding transmissions. For decoding a signal encoded according to a convolutional code, Viterbi decoding with high error correction capability is widely used.
In Viterbi decoding, the actual convolutional code value of a transmitted signal is estimated in the receiver by selecting most probable state transition from a finite set of possible state transitions corresponding to the convolutional code such that the state transition of an encoder can be correctly assumed in connection with reception of the transmitted signal. The above described procedure is commonly referred to as a maximum likelihood decoding procedure. In accordance with maximum likelihood decoding it is common to use the Hamming distance between the received code and a candidate code as a likelihood for quantitatively expressing the probability that candidate code and the received code match in order to determine the correct state transition. When obtaining the likelihood, a hard decision that represents a binary state as a 0 or a 1 of the reception signal and soft decision that represents the reception signal with multiple values according to the amplitude of the reception signal are known.
It should be understood that the result of the hard decision has a direct effect on the decoding output, without regard to the reliability of the hard decision value, whereas a soft decision value has little effect on the decoding output if the likelihood is small. Accordingly the reliability of soft decision decoding is high compared with hard decision decoding. For reference, a decision value as referred to herein will generally represent the likelihood in soft decision decoding.
A communication device is described in JP 2001-257604 A for performing NULL control of a soft decision value in such a way that, when interference occurs, the soft decision value may not contribute to the Viterbi decoding. Note that, in JP 2001-257604 A, it is assumed that a pulse interference wave transmitted from any other system such as, for example, a meteorological radar, or the like, causes interference in the desired wave that communication device should receive. The communication device described in JP 2001-257604 A, detects the occurrence of interference using criteria that the interference wave is periodic and that the transmission power of the interference wave is much larger than a transmission power of the desired wave, or stated differently, the received power difference between the interference wave and the desired wave is large.
Another problem that arises in wireless systems using CSMA/CA access control, such as in vehicle-to-vehicle communication systems, is interference caused by the so-called “hidden terminal” problem.
For example, in the case where there are a plurality of vehicles, such as vehicle A, vehicle B, and vehicle C, each capable of vehicle-to-vehicle communication using CSMA/CA access control, vehicle B or vehicle C can each communicate with the vehicle A, as shown in FIG. 10. Vehicle B and vehicle C are at locations where mutual communication is impossible, and vehicle B is about to transmit data to the vehicle A. Vehicle B performs a carrier sense of a channel used for communication with vehicle A and finds the channel clear. However, vehicle C could be communicating with A and, since vehicle C is out of range of vehicle B, the communication between C and A will not result in a carrier sense by B. Therefore, even if vehicle C is transmitting to vehicle A, vehicle B determines that a channel used for communication with vehicle A is not busy and will transmit data to the vehicle A. Accordingly, interference in the form of the data from the vehicle B can occur in the vehicle A when receiving the desired wave in the form of the data from the vehicle C.
In such a case, interference occurs between signals transmitted from communication devices having the same configuration and constituting one system. Unlike the previously described high power periodic interferers, in the hidden terminal scenario described above, there is little difference in received power between the desired wave and the interference wave and the interference wave is not necessarily transmitted periodically. Therefore, it is difficult to detect hidden terminal interference occurring in the desired wave such as by using the conventional techniques including the device described in JP 2001-257604 A. As a result, it becomes difficult or impossible to correct the soft decision value for the received signal in which hidden terminal interference is occurring, and thereby decode the received data correctly.